If Mox Diamond would enter the battlefield, you may discard a land card instead. If you do, put Mox Diamond onto the battlefield. If you don't, put it into its owner's graveyard.{T}: Add one mana of any color.

Legalities

Rulings

2008-05-01
If you don't discard a land card, Mox Diamond never enters the battlefield. It won't trigger abilities that look for something entering the battlefield, and you won't get the opportunity to tap it for mana.